This invention relates to a laser apparatus and, particularly, to a beam distributor for distributing pumping beam over a laser medium to pump laser active materials doped in the laser medium.
In the fields of optical communication and laser beam machining, it is desired to develop a laser apparatus which is greater in output power and is less expensive. An optical fiber laser apparatus is known for its greater potential to meet the above-mentioned demand.
The optical fiber laser apparatus includes an optical fiber called a laser fiber that comprises a core doped with laser active materials and a clad surrounding (or cladding) the core. The laser active materials are, for example, laser active ions, pigments, or other luminescent materials.
In the optical fiber laser apparatus, a single transverse mode of laser oscillation can relatively easily be achieved by appropriately selecting a diameter of the core and a difference in refractive index between the core and the clad. In addition, by confining a pumping beam within the core at a high density, it is possible to enhance an interaction between the laser active materials and the pumping beam. Furthermore, by increasing the length of the laser fiber, the interaction can be extended so as to produce a high-quality laser beam with a high efficiency. Thus, it is possible by the use of the optical fiber laser apparatus to obtain at a relatively low cost a laser beam excellent in quality.
In order to realize further increase in output power and efficiency of the optical fiber laser apparatus, it is necessary to efficiently introduce the pumping beam into the core. Generally, when the core diameter is determined so as to satisfy a single-mode waveguide condition, it is restricted to a value not greater than ten and several micrometers. Therefore, it is generally difficult to efficiently introduce the pumping beam within the core. To overcome the difficulty, proposal is made of a so-called double-clad fiber laser.
The double-clad fiber laser comprises an optical fiber having a core doped with laser active materials, a first clad surrounding the core and having a first refractive index lower than that of the core, and a second clad cladding the first clad and having a second refractive index lower than the first refractive index. With this structure, a pumping beam introduced into the first clad from the end of the optical fiber is kept confined inside a boundary between the first clad and the second clad during propagation. This is because total internal reflection occurs at the boundary due to the difference in refractive index between the first and the second clads. During the propagation, the pumping beam repeatedly passes through the core and pumps the laser active materials contained in the core. The first clad has a sectional area corresponding to several hundreds to one thousand times that of the core. Therefore, a greater quantity of the pumping beam can be introduced into the optical fiber so as to increase the output power.
Thus, the double-clad fiber laser is advantageous in that the oscillation efficiency is high and that the transverse mode of oscillation is a single mode and stable. Consequently, by using a laser diode as a pumping beam source, the output power of several or about ten watts are obtained in the double-clad fiber laser and are much larger than that of a previous single-clad fiber laser.
However, the double-clad fiber laser is disadvantageous in that the pumping beam source is limited in number because the pumping beam must be introduced into the optical laser at the end of the laser fiber. Thus, there is no way to increase the output power of the double-clad fiber laser except for a way of increasing luminance (or output power) of the pumping beam source (i.e. the laser diode).
In order to overcome the above-mentioned disadvantage, Applicant proposed an optical fiber laser apparatus having a structural member for distributing a pumping beam over a laser fiber, which is directly or indirectly in contact with the structural member. The optical fiber laser apparatus is disclosed in Japanese Unexamined Patent Publication (JP-A) No. H11-284255. Hereinafter, the structural member will be referred to as a beam distributor.
The beam distributor has a main body having a hollow or solid cylindrical shape and a prism formed at an end surface of the main body. If the pumping beam is introduced at a predetermined angle in the beam distributor through an incident surface of the prism, it is confined within the main body by the total internal reflection at its surfaces. When a laser fiber is coiled around the beam distributor, the pumping beam confined in the beam distributor is introduced into the laser fiber. This is because the necessary conditions of the total internal reflection do not be met at a contact area where the beam distributor and the laser fiber are in contact with each other. The pumping beam introduced in the laser fiber pumps laser active materials included in a core of the laser fiber. Thus, in the optical fiber laser apparatus, pumping is equally made at a long range of the laser fiber and the laser fiber efficiently generates a laser beam.
By the way, the beam distributor decreases the pumping beam during propagation within the beam distributor. That is, the pumping beam is partially lost by propagation loss in the beam distributor and escape loss from the incident surface. If the total of the propagation loss and the escape loss in the beam distributor is large, the pumping beam is inefficiently introduced into the laser fiber. Therefore, it is desirable to make small both of the propagation loss and the escape loss.
The propagation loss can be suppressed by the use of a quartz glass that has high transparency while the escape loss can be suppressed by making small the incident surface.
However, when the incident surface is small, it is necessary to condense the pumping beam at the incident surface. As a result, power density of the pumping beam inevitably increase at the incident surface. If the incident surface is stained with dust, the dust is scorched by the condensed pumping beam and stuck on the incident surface. The stuck dust on the incident surface not only prevents the pumping beam from being introduced into the beam distributor also is heated by the pumping beam very much. The heat of the stuck dust partially damages the incident surface.
It is therefore an object of this invention to provide a beam distributor which is capable of being introduced with large quantity of pumping beam.
It is another object of this invention to provide a beam distributor which is capable of efficiently introducing a pumping beam into a laser medium.
It is still another object of this invention to provide a beam distributor which is difficult to be influenced by dust on an incident surface.
It is yet another object of this invention to provide a laser apparatus which comprises a beam distributor capable of being introduced with large quantity of pumping beam.
It is a further object of this invention to provide a laser apparatus which comprises a beam distributor capable of efficiently introducing a pumping beam into a laser medium.
It is a still further object of this invention to provide a laser apparatus which comprises a beam distributor difficult to be influenced by dust on an incident surface.
Other object of this invention will become clear as the description proceeds.
According to the gist of this invention, a beam distributor is used in a laser apparatus to distribute a pumping beam supplied from a pumping beam source over a laser medium. The beam distributor comprises a beam conductor which has an incident surface and an intermediate surface and which conducts the pumping beam received by the incident surface to the intermediate surface. A main body is connected to the intermediate surface and has a plurality of surfaces. The main body confines the pumping beam introduced therein through the intermediate surface, by repeating total internal reflection at the surfaces, to distribute the pumping beam confined therein over the laser medium where the laser medium is in contact with the surfaces of the main body.
According to another gist of this invention, a laser apparatus comprises a pumping beam source which produces a pumping beam. A laser medium is pumped by the pumping beam to emit a laser beam. A beam distributor distributes the pumping beam over the laser medium to pump the laser medium. The beam distributor comprises a beam conductor which have an incident surface and an intermediate surface and which conducts the pumping beam received by the incident surface to the intermediate surface. A main body is connected to the intermediate surface and has a plurality of surfaces. The main body confines the pumping beam introduced therein through the intermediate surface, by repeating total internal reflection at the surfaces, to distribute the pumping beam confined therein over the laser medium where the laser medium is in contact with said surfaces of the main body.
According to still another gist of this invention, a laser unit has a laser medium to emit a laser beam in response to a pumping beam supplied from a pumping beam source. The laser unit comprises a beam conductor which has an incident surface and an intermediate surface and which conducts the pumping beam received by the incident surface to the intermediate surface. A main body is connected to the intermediate surface and has a plurality of surfaces. The main body confines the pumping beam introduced therein through the intermediate surface, by repeating total internal reflection at the surfaces, to distribute the pumping beam confined therein over the laser medium where the laser medium is in contact with the surfaces of the main body.